Lithium-ion secondary batteries have high charged and discharged capacities, and are batteries being able to make the outputs high. Currently, the lithium-ion secondary batteries have been used mainly as power sources for portable electronic appliances, and have further been expected as power sources for electric automobiles anticipated to become widespread from now on. The lithium-ion secondary batteries comprise active materials being capable of inserting and eliminating (or sorbing and desorbing) lithium (Li) in the positive electrode and negative electrode, respectively. And, lithium ions moving within an electrolytic solution disposed between the two electrodes lead to operating the lithium-ion secondary batteries.
In the lithium-ion secondary batteries, a lithium-containing metallic composite oxide, such as lithium/cobalt composite oxides, has been used mainly as an active material of the positive electrode; whereas a carbon material having a multilayered structure has been used mainly as an active material of the negative electrode. The performance of the lithium-ion secondary batteries is dependent on materials of the positive electrode, negative electrode and electrolyte constituting the secondary batteries. Even among the materials, researches and developments of active-material ingredients forming the active materials have been carried out actively. For example, silicon or silicon oxides having a higher capacity than the capacity of carbon have been investigated as a negative-electrode active-material ingredient.
Using silicon as a negative-electrode active material enables a battery to have a higher capacity than using a carbon material. However, silicon exhibits a large volumetric changes accompanied by occluding and releasing (or sorbing and desorbing) lithium (Li) at the time of charging and discharging operations. Consequently, silicon has been pulverized finely to cause structural changes so that silicon has peeled off or come off from a current collector, and thereby such a problematic issue arises probably that the charging/discharging cycle longevity of a battery is short. Hence, using a silicon oxide as a negative-electrode active material enables the volumetric changes accompanied by sorbing and desorbing lithium (Li) at the time of charging and discharging operations to be inhibited more than using silicon.
For example, employing as a negative-electrode active material silicon oxide (e.g., SiOx where “x” is 0.5≤“x”≤1.5 approximately) has been investigated. The SiOx has been known to decompose into Si and SiO2 when being heat treated. The decomposition is referred to as a “disproportionation reaction,” the SiOx separates into two phases, an Si phase and an SiO2 phase, by the internal reactions of solid. The Si phase separated to be obtainable is very fine. Moreover, the SiO2 phase covering the Si phase possesses an action of inhibiting electrolytic solutions from being decomposed. Therefore, a secondary battery, which uses a negative-electrode active material composed of the SiOx having been decomposed into Si and SiO2, excels in the cyclability.
The finer silicon particles constituting the Si phase of the above-mentioned SiOx are, the more a secondary battery using the particles as a negative-electrode active material is upgraded in the cyclability. Hence, Japanese Patent No. 3865033 (i.e., Patent Application Publication No. 1) sets forth a process for producing SiOx by heating metallic silicon and SiO2 to sublime in order to turn the metallic silicon and SiO2 into a silicon oxide gas, and then cooling the silicon oxide gas. The process enables the particle diameters of the silicon particles constituting the Si phase to exhibit such a nanometer size as from 1 nm to 5 nm.
Moreover, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2009-102219 (i.e., Patent Application Publication No. 2) sets forth a production process in which a silicon raw material is decomposed down to the elemental states in a high-temperature plasma, the decomposed silicon raw material is cooled quickly down to a liquid nitrogen temperature to obtain nanometer-size silicon particles, and the nanometer-size silicon particles are fixated into an SiO2—TiO2 matrix by a sol-gel method, and the like.
However, according to the production process set forth in Patent Application Publication No. 1, the matrix is limited to subliming materials. Moreover, according to the production process set forth in Patent Application Publication No. 2, a high energy has come to be needed for plasma discharge. In addition, the silicon composites obtained by the production processes have such a drawback that the dispersibility of Si-phase silicon particles is so low that the particles are likely to agglomerate. When the Si particles agglomerate one another so that the particle diameters become large, a secondary battery using the agglomerated Si particles as a negative-electrode active material has a low initial capacity, and the cyclability also declines.
Incidentally, nanometer-size silicon materials, which have been expected to be utilized in fields such as semiconductors, electric and electronic engineering, have been developed in recent years. For example, Physical Review B(1993), vol. 48, pp. 8,172-8,189 (i.e., Non-patent Literature No. 1) sets forth a process in which a lamellar polysilane is synthesized by reacting hydrogen chloride (HCl) and calcium disilicide (CaSi2) one another. The article further sets forth that the thus obtained lamellar polysilane is utilizable for light-emitting devices, and the like.
And, Japanese Unexamined Patent Publication (KOKAI) Gazette No. 2011-090806 (i.e., Patent Application Publication No. 3) sets forth a lithium-ion secondary battery using a lamellar polysilane as a negative-electrode active material.